Cable signal distribution systems require the distribution of many channels of signals with precise control of the characteristics for each channel within specified limits, as well as the selected inhibition of one or more of those signal channels. The problems of such wideband signal distribution, while formidable in themselves, are further exascerbated by the requirements to selectively inhibit or scramble premium security channels. Each channel control must necessarily be limited to the specific channel to be deleted, while preventing the effect of the deletion or distortion of the channel to the adjacent channels or subscriber positions.
Restricted access to signals is effectively provided either by their complete removal or encoding or scrambling so as to render them unintelligible for the particular subscriber application. Many of the schemes of encoding require the signal to be encoded at the head end and each subscriber to have a corresponding signal decoder in their home receiver location. Systems so constructed incur costs both of the encoder and of each decoder. Moreover, a signal must include the key necessary to provide the signal decoding in order for the information to be recovered. Such keys may be recovered by unauthorized users to provide unauthorized access to the protected cable signals.
Other means of signal encryption include the addition of jamming signals to obscure the desired signal information. However, the addition of nonbroadcast jamming signals must be carefully controlled to remain within limits of tolerable signal radiation. Moreover, the jamming signal produced easily generates components outside of the desired band of interest, which when mixed with other jamming or broadcast signal components, may produce extraneous signals which produces undesirable interference on other channels, as well as feeding back through the system to other signal subscribers. Furthermore, if the amplitude of the protected cable signal increases, the jamming signal may no longer be sufficientto obscure that signal.
The technique of restricted access to signals by attenuating the signal requires careful attenuation control at the band of interest. If the band of rejection is excessively broad or not properly tuned, it will interfere with the adjacent, wanted signals. Alternately, if the band of rejection is unnecessarily narrow, enough of the protected signal may pass without attenuation to provide a usable signal to the unauthorized user. Furthermore, the precise control of filter circuits to provide selective rejection of cable signals, particularly at the superband (near UHF) cable channels, and above, requires a circuit more precisely adjustable than lumped resonant circuits, which suffer from problems from the drift, mistuning, and mechanical and thermal changes in the elements. Moreover, since the resonant circuits in the higher frequency bands have a limited Q, they cannot achieve the desired rejection necessary to provide adequate signal rejection while avoiding the adjacent channel interface, multiple resonant sections must be cascaded to provide the desired filtering characteristics. This multiple tune circuit structure further aggravates the tuning and stability problems of the filter structure.